On Mon, Nov 3, 2025 at 2:26 PM Adrian Vogelsgesang via Gcc <[email protected]>
wrote:
> And I just realized that I inadvertently dropped the ball by replying
> only to Sandoe, and forgot to keep the list in CC.
>
> Thanks for the quick response, Iain!
> Inline below my (original, by now pretty late) replies
>
> > > There are two reasons the script doesn't work for g++:
> > > 1. g++ does not emit a `__promise` variable inside the destroy
> > > function - that can be worked around by changing the script, though
> >
> > g++ accesses via the frame pointer - the following entries should be
> > available to the debugger.
> >
> > frame_pointer->_Coro_promise
> > frame_pointer->_Coro_resume_index
>
> Yes, that can be easily fixed in the script - I just didn't implement it,
> yet.
>
> > > 2. g++ provides no way to map the compiler-assigned suspension point
> > > IDs back to line numbers - i.e., the topic of this email
> >
> > g++ retains the location information for user-authored code (so that
> setting
> > breakpoints on line nunber etc. should work.)
>
> yes, and that works great! However, that is besides my point / solving a
> different problem.
>
> In my case, I have a suspended `std::coroutine_handle` which I want to
> pretty-print. From that coroutine_handle, I can get
> 1. a pointer to the destroy function's entry point (e.g., 0x7f1345)
> 2. the resume index (e.g., 15)
>
> I now need a way to map `0x7f1345` and `idx 15` back to its location.
> I.e., I need to answer "the suspension point with id 15 within the
> coroutine whose destroy function starts at address 0x7f1345 is located
> in foobar.cpp at line 65".
>
> Note that I cannot simply lookup 0x7f1345 in the line table, since that
> would give me the start of the coroutine function, not the position of
> suspension point 15 inside the function's body.
>
> In clang, I solved this by:
> 1. lookup the scope of the destroy function:
> destroy_func = gdb.block_for_pc(int(self.destroy_ptr))
> 2. lookup the label for suspension point 15 within that function
> label_name = f"__coro_resume_{suspension_point_index}"
> resume_label = gdb.lookup_symbol(label_name, self.resume_func,
> gdb.SYMBOL_LABEL_DOMAIN)[0]
> 3. look at that label's line/column
> print(f"suspended at line {resume_label.line}")
>
> For gcc-compiled code
> 1. step 1 also works
> 2. step 2 works with a small adjustment to the label name
> 3. step 3 does not work, since the labels produced by gcc have
> neither location information nor a DW_AT_low_pc which I could look
> up in the line table.
>
> > However, for synthetic code (e.g. the ramp and the expansion of the
> > co_await expressions) so far, we have intentionally generated the code
> > with “unknown” locations. This (absent the kind of process you are
> > mentioning) tends to impove the debug experience - because it avoids
> > the apparent location jumping around.
>
> That's great! clang unfortunately does emit debug info for this syntetic
> code, and hence single stepping into / out of a clang-compiled coroutine
> is a bit clunky
> On Fri, Oct 10, 2025 at 8:19 AM Iain Sandoe <[email protected]>
> wrote:
> >
> >
> >
> > > On 10 Oct 2025, at 03:16, Adrian Vogelsgesang via Gcc <[email protected]>
> wrote:
> > >
> > > Hi gcc-devs!
> > >
> > > TLDR: For debugging C++ coroutines, I patched clang to emit artificial
> > > DW_TAG_labels, mapping suspension point ids to the corresponding
> > > source location. Looking for alignment re debugging info between clang
> > > and gcc.
> > >
> > > (Finally following up on Iain Sandoe's request to send this email)
> >
> > Thanks, there are others here much better able to comment on debug info
> > than I am. I’ve added a couple of notes below but hope that others will
> > chime in with opinions on how to proceed.
> >
> > > ~~~~~~~~~
> > > Background
> > > ~~~~~~~~~
> > >
> > > When using coroutines for asynchronous programming, the physical stack
> > > only tells part of the truth. One also wants to see the chain of
> > > "awaiting" coroutines, i.e. the coroutines which initiated the current
> > > coroutine frame and are waiting for its completion.
> > >
> > > I published a gdb debugger script which provides a FrameFilter to
> > > inject those async stack frames. With this script, the `bt` command
> > > now returns
> > >
> > >> #0 write_output(...) at ...
> > >> [async] greet() at ...
> > >> [async] [noop_coroutine] at ...
> > >> #1 coroutine_handle<task::promise_type>::resume() const at ...
> > >> #2 task::syncStart() at ...
> > >
> > > However, this script currently doesn't work for gcc-compiled binaries,
> > > yet, due to missing debug information.
> > >
> > > ~~~~~~~~~
> > > Current state of gcc-generated debug info
> > > ~~~~~~~~~
> > >
> > > There are two reasons the script doesn't work for g++:
> > > 1. g++ does not emit a `__promise` variable inside the destroy
> > > function - that can be worked around by changing the script, though
> >
> > g++ accesses via the frame pointer - the following entries should be
> > available to the debugger.
> >
> > frame_pointer->_Coro_promise
> > frame_pointer->_Coro_resume_index
> >
> > The resume index is updated as we pass the test for the awaiter being
> > ready - so that it should be correct whether the coroutine suspends or
> > continues.
> >
> > > 2. g++ provides no way to map the compiler-assigned suspension point
> > > IDs back to line numbers - i.e., the topic of this email
> >
> > g++ retains the location information for user-authored code (so that
> setting
> > breakpoints on line nunber etc. should work.)
> >
> > However, for synthetic code (e.g. the ramp and the expansion of the
> > co_await expressions) so far, we have intentionally generated the code
> > with “unknown” locations. This (absent the kind of process you are
> > mentioning) tends to impove the debug experience - because it avoids
> > the apparent location jumping around.
> >
> > > In clang, I solved this issue by emitting DW_TAG labels like
> > >
> > >> 0x00000f71: DW_TAG_label
> > >> DW_AT_name ("__coro_resume_17")
> > >> DW_AT_decl_file ("generator-example.cpp")
> > >> DW_AT_decl_line (5)
> > >> DW_AT_decl_column (3)
> > >> DW_AT_artificial (true)
> > >> DW_AT_LLVM_coro_suspend_idx (0x11)
> > >> DW_AT_low_pc (0x00000000000019be)
> > >
> > > The debugging script can lookup the DW_TAG_label for a given
> > > suspension point either by name or via DW_AT_LLVM_coro_suspend_idx and
> > > retrieve the line, column and address (for setting breakpoints) from
> > > that label.
> > >
> > > gcc emits similar information:
> > >
> > >> 0x0000297c: DW_TAG_label
> > >> DW_AT_name ("resume.17")
> > >>
> > >> 0x00002981: DW_TAG_label
> > >> DW_AT_name ("destroy.17")
> > >
> > > Unfortunately, this information is not useful because it lacks file,
> > > line, column and address information. It would be great if g++ could
> > > also emit file, line, column and address information for those labels.
>
It looks like those labels internally already have the location
information, but the DWARF writer decides not to represent it because the
labels are marked DECL_ARTIFICIAL. We might just remove that flag from
create_named_label_with_ctx (even though they are indeed artificial)?
> > > ~~~~~~~~~
> > > Can gcc also emit useful DW_TAG debug information for coroutines?
> > > ~~~~~~~~~
> > >
> > > What do you think about the approach of using DW_TAG_label for
> > > debugging coroutines? Would you be willing to adopt the same approach
> > > also for g++? (I would also be happy to adjust clang, in case we come
> > > to a different alignment between both compilers).
> > >
> > > Adding the file, line, column and address would probably be pretty
> > > fundamental for a good debugging experience. Also it would be nice
> > > (although completely optional) if we could use the same naming
> > > convention (`__coro_resume_x`) and you might want to set the
> > > DW_AT_artificial tag. I chose `__coro_resume_x` for clang, because
> > > this is a reserved name which is still easily writeable in debugger
> > > commands. Using the DW_AT_artificial for those labels also seems to
> > > make semantically sense (although it is strictly speaking not blessed
> > > by the DWARF standard).
> > >
> > > ~~~~~~~~~
> > > Further Reading
> > > ~~~~~~~~~
> > >
> > > RFC for LLVM/clang:
> > >
> https://discourse.llvm.org/t/rfc-debug-info-for-coroutine-suspension-locations-take-2/86606
> > >
> > > Corresponding clang commit:
> https://github.com/llvm/llvm-project/pull/141937
> > >
> > > Background on debugging of coroutines, both from the user's point of
> > > view and toolchain implementation details, such as the approach for
> > > devirtualizing the coroutine frame's state:
> > > https://clang.llvm.org/docs/DebuggingCoroutines.html
> > >
> > > Best,
> > > Adrian
> >
>
>
